brcmfmac: remove obsolete i-scan and clean up related code.

e-scan has become the default scanning method. This patch removes
the i-scan related code and cleans up e-scan related code to be
always enabled.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>